Sunn Beta Bias problem

[walter]

I am working on a Sunn Beta amp. When I got it the output was just a hum. The output devices check good. I tried a few things and then powered it on without the heatsink connected to the chassis, now there is no output. It looks like the ground of the circuitboard connects to the chassis through the heatsink. I rechecked the output devices, they check good. It looks like an imbalance in the bias circuit around Q13. Q13E and Q14C should be -1.7, I see 40.81v. Makes me suspect Q14. I’ve tried a bunch of things including replacing Q14.
Does anything look off, what can I try to pinpoint the fault? I have attached a schematic with voltages. Values in black are good, red are bad. This isn’t a very clear schematic, so here is a schematic of the PA. I tried to paste voltages on the PA schematic, but it wouldn’t save…
Any help is much appreciated. I have another working Beta to compare.

 

[walter]

Here is the clearer schematic.

 

[JohnRoberts]

the voltages you measure at q14 are not enough to turn q14 on. You show two different voltages for the same node -45 and -45.18, confirm that collector q12 is connected to base of q14

JR

 

[abbey road d enfer]

I have another working Beta to compare.

Looks like Q7 is duff. 40+ volts between base and emitter is not right. Maybe the three others (Q8-9-10)could be duff too.

 

[CJ]

with the Beta series, bass or lead, i put in some TIP devices and mount them on the chassis and run 6 wires out there,

those TO-220 pre drivers run hotter than heck, Sunn did not heatsink them enough.

do that and it will run forever.

i think i use TIP 31 and it's compliment.

also check to see if there is a voltage regulator in a TO-220 that is gone bad,

Deox the cable connector, spray the pots, sand down the bottom of the heat sink, use no grease and get ready to rock!

 

[CJ]

working on a beta lead right now,

notice burned pc board under driver>

 

[CJ]

Sunn usually leaves a lot of overspray on the chassis under the sink, so we take that off>

 

[CJ]

looks like somebody been in here before, so we will go through the hole board.  :

 

[CJ]

good idea to reflow the pwr trans pins,

some look funky.

have to "hit and git" with the iron so they do not dislodge.

 

[CJ]

we checked the main filters, they are good,

we changed out all the 10/50 caps,

we re-flowed the power transistor solder joints,

opamp socket gets to stay

we removed all the socketed transistors, checked them, removed the sockets and resoldered the transistors

since these amps sometimes sit on top of a vibrating speaker cabinet, we want a solid mechanical connection.

power transistors were checked for hfe, NPN 3055's were good,

PNP pr transistors were of two different part numbers so we found some old school PNP's of the same number and matched them up,

we will now locate the too hot pre-drivers off the board and test this thing,

if it is good, we will spray the pots and bolt er up,

 

[JohnRoberts]

I recall Conrad Sundholm doing some interesting (IMO good) packaging concepts back in the 80s but reliability engineering was not his strong suit.

We all start ignorant... some die ignorant, some don't.

JR

 

[CJ]

ok we have Q1- TIP29 and Q11-TIP30 mounted to separate heatsinks and insulated from the chassis so we do not have to use an insulator kit.


these are pass transistors for +/- 15V regulation, been a whaile, i thought they were drivers stupid me, 

 

[walter]

Some good suggestions here. The different voltages at the same node are probably my sloppy data collecting.

the voltages you measure at q14 are not enough to turn q14 on.

JR

This is the kind of help I need, I suspected this might be the case, but I don’t have the intuition to see this.
I started by running a signal in and scoping. I wasn’t getting signal to the output, so I tried comparing voltages. I have measured resistances, checked diodes, replaced a few transistors, measured continuity between components. I decided to run signal again, and the bias imballance went away. I get good signal to R37 and R12, nothing at R16 and R46. So Q7 may be a problem.
This unit too has a burned spot under R25, the 5Watt resistor that supplies the TIP30 pass through device at Q11 for the -15v supply. There is more curent on the -15v supply than the +15v side since the -15v powers the leds and the CMOS channel switching logic.

we removed all the socketed transistors, checked them, removed the sockets and resoldered the transistors

since these amps sometimes sit on top of a vibrating speaker cabinet, we want a solid mechanical connection.

I agree, the transistor sockets can cause problems, I remove them too.

I’ll keep poking around Q7 and Q15 when I can find some time to get back to this.

 

[JohnRoberts]

Perhaps another measurement error but 40+ V between base emitter of Q7 is no  bueno.. as Abbey pointed out.

Sounds like CJ spent some time under the hood of these so has some first hand advice, better than my guesses.

JR

 

[CJ]

good idea to touch up the spider pots,

hey, i thought those were exclusive to Peavey?

solder did not wick up through the holes,

these post hold the circuit board, so the mechanical stress can compromise the electrical connection, you can sometimes see stress cracks on these pot solder joints,

we replaced the 3 big filters on this pramp board but not the 1 zillion 2.2 uf NP  caps.

 

[JohnRoberts]

good idea to touch up the spider pots,

hey, i thought those were exclusive to Peavey?

Sorry...

As I recall those spider pots were the same height as hi-d (1/4") jacks  (11/16" IIRC but that was a long ass time ago). Handy technology for large mixers on single flat PCB.


JR 

solder did not wick up through the holes,

these post hold the circuit board, so the mechanical stress can compromise the electrical connection, you can sometimes see stress cracks on these pot solder joints,

we replaced the 3 big filters on this pramp board but not the 1 zillion 2.2 uf NP  caps.

 

[walter]

I made a signal injector to help me determine what is working. A capacitor taped to the end of a chopstick with spring clip wired to one end. I get signal to R37 and R12 (the base of Q7 and Q16), so connect the clip there. Probe with the other end to see where the output works. The Emitter of Q7 or Q16, this amp is loud! So Q7 and Q16 are not turning on.
I found a discussion about a Sunn amp where someone connected the power up backwards. Someone else had several suggestions and they got it working again. From that I read:
“Make sure the bias xstr Q13 is still good. If it is now open circuit, Q5 and Q14 voltage gain stage is probably turned on hard, pulling pre-drivers Q7 & Q16 on hard, which would turn drivers Q8 & Q17 on hard...enough to cause your R17 to burn. If you jumper across the C-E junction of the bias xstr, it would basically turn off the output stage. The bias xstr is normally adjusted to control just how much voltage you want at the Vbe junctions of Q8 & Q17. That voltage is normally less than the normal junction voltage of about 0.6V or so. 0.35V to 0.5V is probably in the range Sunn sets it up at. That voltage is set by the trim pot R42.
So, first, verify all the rest of your circuit semi's are good, and if any resistors look like they've burned open, replace them. The cap C13 that's in the burn region is a signal bypass path around the bias xstr Q13.

It's a very simple & basic complimentary solid state power amp."

This amp I’m working on, going from that statement, looks like the bias is shorted, causing Q5 and Q14 to turn off, and in turn, not supplying current to turn on Q7 and Q16. I thought C13 might be short, so I replaced that, I also replaced Q13, originally 2N4250 I used 2SB737. The resistors measure similar to my other amp that works. The bias pot also seems to be good. There is something going on in this area. I did remove the sockets for the transistors, Q6, Q15, and Q16. Now the imballace is negative, with -35v at the base of Q7, and -43 volts at the base of Q16 (also collector of Q14) so now it looks like Q14 is shorted, or full on, where before it looked like Q5 was short or full on.
I’m going to try something unless anyone has another suggestion. I can remove Q7 and Q16. Remove the bias Q13 so Q5 and Q14 turn on hard, and in turn should turn on Q7 and Q16. I’ll then check voltages, and run signal to scope through that part of the circuit. Hopefully the culprit will become obvious.

 

[JohnRoberts]

I made a signal injector to help me determine what is working. A capacitor taped to the end of a chopstick with spring clip wired to one end. I get signal to R37 and R12 (the base of Q7 and Q16), so connect the clip there. Probe with the other end to see where the output works. The Emitter of Q7 or Q16, this amp is loud! So Q7 and Q16 are not turning on.
I found a discussion about a Sunn amp where someone connected the power up backwards. Someone else had several suggestions and they got it working again. From that I read:
“Make sure the bias xstr Q13 is still good. If it is now open circuit, Q5 and Q14 voltage gain stage is probably turned on hard, pulling pre-drivers Q7 & Q16 on hard, which would turn drivers Q8 & Q17 on hard...enough to cause your R17 to burn. If you jumper across the C-E junction of the bias xstr, it would basically turn off the output stage. The bias xstr is normally adjusted to control just how much voltage you want at the Vbe junctions of Q8 & Q17. That voltage is normally less than the normal junction voltage of about 0.6V or so. 0.35V to 0.5V is probably in the range Sunn sets it up at. That voltage is set by the trim pot R42.
So, first, verify all the rest of your circuit semi's are good, and if any resistors look like they've burned open, replace them. The cap C13 that's in the burn region is a signal bypass path around the bias xstr Q13.

It's a very simple & basic complimentary solid state power amp."

This amp I’m working on, going from that statement, looks like the bias is shorted, causing Q5 and Q14 to turn off, and in turn, not supplying current to turn on Q7 and Q16. I thought C13 might be short, so I replaced that, I also replaced Q13, originally 2N4250 I used 2SB737. The resistors measure similar to my other amp that works. The bias pot also seems to be good. There is something going on in this area. I did remove the sockets for the transistors, Q6, Q15, and Q16. Now the imballace is negative, with -35v at the base of Q7, and -43 volts at the base of Q16 (also collector of Q14) so now it looks like Q14 is shorted, or full on, where before it looked like Q5 was short or full on.
I’m going to try something unless anyone has another suggestion. I can remove Q7 and Q16. Remove the bias Q13 so Q5 and Q14 turn on hard, and in turn should turn on Q7 and Q16. I’ll then check voltages, and run signal to scope through that part of the circuit. Hopefully the culprit will become obvious.

It is usually easier to troubleshoot simple amp circuits by inspecting active device junction voltages.

With transistors you can generally confirm the health of the device from valid junction voltage drops, and even device on/off status from Vbe . <<one diode drop Vbe is off, << one diode drop is broken.

You started out with schematic marked up with measured voltages. We suggested several voltages that appeared worth looking into. That is where to begin looking.

JR

 

[walter]

Ah, o.k. Thanks. I don’t have a lot of experience with solid state. It looks like I need to remove the active components to measure. I thought Q5 might be bad since there seemed to be no voltage drop. I replaced that, but it didn’t change. I then removed Q7 and there was still 44v at the collector of Q5. Then I replaced Q13, Q14, and Q12 one at a time with no change. At that point I suspected C13 could be short, so I replaced that. Still problems. I then replaced Q7 and looked at signal on a scope. That is when I decided to start this discussion. I took another look at Q7 last night and it was bad, so I put another new device there. Now I get signal through the amp, but the bias is still off. I see -8v at the base of Q7, and -12v at the base of Q16 and the signal looks like a cowboy hat at those points. I’ll keep at this until I get it 100%. Then I can move on to the next one. The next one sounds good, but has several broken knobs, pot shafts. Where can I get pots and knobs for these?

 

[scott2000]

Where can I get pots and knobs for these?

Maybe the Alpha RV24AF-40 could work?
https://www.mouser.com/Search/Refine?Keyword=RV24AF-40

Not sure of the dimensions you need... But they were a pretty good size iirc....